A conventional fusing device of a roller method, employed in an electrophotographic apparatus, such as an electrophotographic copying machine, an electrophotographic facsimile, and an electrophotographic printer, comprises a fusing roller and a pressing roller pressed against the fusing roller, whereby an image is fused onto a recording medium as the recording medium is carried through a section between the pair of rollers while one or both of the rollers are being heated.
In the fusing device of the roller method, however, the pair of rollers must be rotated synchronously. In addition, each roller must be supported so that it can rotate, which complicates the structure of the device and hence increases the size and manufacturing costs. To eliminate these problems, Japanese Examined Patent Publication No. 36996/1980 discloses a pressing pad method. More precisely, a non-rotational pressing member is pressed against the fusing roller instead of the pressing roller, whereby an image is fused onto a recording medium as the recording medium is carried through a section between the fusing roller and pressing member. Also, Japanese Laid-Open Patent Publication No. 304481/1989 discloses a pressing sheet method. More precisely, a pressing web member is pressed against the fusing roller at a predetermined contact angle instead of the pressing roller, whereby an image is fused onto a recording medium as the recording medium is carried through a section between the fusing roller and pressing web member.
An example fusing device of the pressing pad method will be explained with reference to FIG. 20. A fusing roller 112 comprises an aluminium hollow roller 112a whose outer circumference surface is coated with a coat layer 112b which is silicon rubber or the like having a large friction coefficient. A pressing member 111 is provided below the fusing roller 112, and the surface thereof opposing the fusing roller 112, namely, the pressing surface, is coated with a coat layer 114 made of, for example, polytetrafluoroethylene resin having a small friction coefficient. The pressing member 111 is fixed onto an upper surface of a pressing plate 116 supported by an axis 117, and pressed against the fusing roller 112 by a pressing spring 118 under a predetermined pressure. A pre-fused toner image 102 transferred onto a paper 101 is fused thereon as the paper 101 is carried through a section between the fusing roller 112 and pressing member 111.
Next, an example fusing device of the pressing sheet method will be explained with reference to FIG. 21. A fusing roller 122 comprises an aluminium hollow roller 122a whose outer circumference surface is coated with a coat layer 122b which is silicon rubber or the like having a large friction coefficient. A pressing web member 121, which is engaged with a frame 123 at one end and stretched out by a coil spring 128 with a predetermined tensile force at the other end, is pressed against the fusing roller 122 at a predetermined contact angle a. A pre-fused toner image 102 transferred onto a paper 101 is fused thereon as the paper 101 is carried through a section between the fusing roller 112 and pressing web member 121.
However, the fusing device of the pressing pad method has following problems:
1 Since the adhesion strength of the coat layer 114 to the pressing member 111 is weak, the coat layer 114 readily wears and separates from the pressing member 111 as the fusing roller 112 slides thereon. PA1 2 Since a shear force develops and acts on the pressing member 111 when the fusing roller 112 rotates while no paper 101 is carried through the section between the fusing roller 112 and pressing member 114, for example, in case of a preliminarily rotation or intervals of continuous paper feeding, the pressing member 111 deforms as shown in FIG. 22. PA1 3 When the pressing member 111 and pressing plate 116 are bonded together in a larger area to increase the adhesion strength, the size of the pressing member 111 and hence the manufacturing costs are undesirably increased. PA1 4 If the pressing member 111 is too rigid, a sufficient nip width can not be secured and a toner image is fused unevenly in a longitudinal direction of the fusing roller 112. Whereas if the pressing member 111 is too soft, there readily occurs permanent deformation. PA1 5 Since the pressing member 111 is fixed to the pressing plate 116, a paper is carried forward through a carrying force of the fusing roller 112 alone, which reduces the readiness in carrying papers and causes frequent paper jam. PA1 6 If the coat layer 112b of the fusing roller 112 alone is to realize conflicting benefits, namely, mold lubrication and readiness in carrying papers, the carrying force must be increased considerably to an optimum level. PA1 1 The contact angle a of the pressing web member 121 with respect to the fusing roller 122 must be increased to obtain a sufficient fusing force (fusing strength), which, in turn, curls the post-fused paper 101 more than necessary. PA1 2 Since the pressure of the pressing web member 121 is distributed unevenly to the fusing roller 122 in the longitudinal direction thereof, a toner image is often fused unevenly. PA1 a fusing roller; PA1 a pressing device made of a resilient material, the pressing device being pressed against an outer circumference surface of the fusing roller under an even pressure; and PA1 a heat-resistant sheet composed of a substrate made of a glass fiber, the heat-resistant sheet being placed on a surface of the pressing device brought into contact with the fusing roller, PA1 a recording material being carried through a section between the fusing roller and heat-resistant sheet as the fusing roller rotates.
Also, the fusing device of the pressing sheet method has the following problems:
In addition, the fusing devices of both the pressing pad method and pressing sheet method have a common problem. More precisely, in case of two-side printing or the like where a second toner image is fused onto a paper having recorded a first toner image on the back side thereof (hereinafter, referred to as used paper), the first toner image may be smeared (hereinafter, referred to as smeared image) as the back side of the used paper slides on the pressing member 111 or pressing web member 121, or there may occur frequent paper jam as the toner on the back side melts and adheres to the pressing member 111 or pressing web member 121.